The parcel shipment industry is increasing the pace of automation in its package-handling and warehousing requirements to reduce delivery time and cost of handling. One such initiative includes the use of machine vision systems (optical readers) to automatically scan packages, without operator intervention, to read the package applied bar code which identifies the package routing, destination, and shipper/addressee. This application's challenge, however, is the necessarily high accuracy standard required to prevent misinformation in the shipping data despite the random depth of field requirements and random placement of the bar code within the camera's field-of-view due to variation in package size and placement on a conveyor belt.
In a typical application, the camera must focus on packages ranging in height from 0-36 inches with an image resolution suitable to read a 10 mil (0.010 inch; approximately 0.25 mm) character font bar code. Prior art vision systems meet these requirements by using vision equipment which measures the parcel height at an upstream position on the conveyer belt. The sensed height data is then used to command the position of a zoom lens of a dynamic focus camera placed downstream, which mechanically positions its lens to capture a focused bar code image. This, however, requires the use a camera with moving mechanical parts.
For improved operational reliability and speed of response it is preferred to use a static focus system which satisfies the range and resolution requirements without dynamic motion.